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# Find Critical Mach number graphically

Asked by Kyle on 12 Feb 2013

Hey,

I have to plot two curves to find the critical mach number in matlab. Very rusty at Matlab (been a few years). For the first curve I have to plot the Critical Pressure coefficient value vs. the Critical Mach number and the other curve is the Pressure coefficient (calculated with the minimum pressure coefficient at low speed) vs object mach number

my code thus far is:

Cpo = -0.65; Mcr = linspace(1,8,20); Minfin = linspace(0,1,20);

Ccrit = (2/(1.4*Mcr.^2))*(((((2+(.4*Mcr.^2))/2.4))^3.5)-1); Cp = Cpo/(sqrt(1-Minfin.^2));

plot(Mcr,Ccrit) plot(Minfin,Cp) grid

and I keep getting an error in the Ccrit and Cp lines. Any help is appreciated thanks

Brian B on 12 Feb 2013

No worries. But there's nothing like a little debugging to refresh the memory. I'm not trying to be annoying; this is how I learned MATLAB, too. (And I'm doing my own homework right now, as well.)

In the error

` Error using /`

you'll notice that the slash is bold and underlined. Click on it. Compare with the function rdivide.

Kyle on 12 Feb 2013

Sorry but still don't completely understand where the error is completely

Brian B on 12 Feb 2013

You've probably figured it out by now, but you need to change use

``` Ccrit = (2./(1.4*Mcr.^2)) ...*(((((2+(.4*Mcr.^2))/2.4)).^3.5)-1);
%         ^                                            ^
%         |                                            |
% Note the periods before the "/" and before the "^".```

You will need to make a similar change to the equation for Cp.

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Answer by Teja Muppirala on 12 Feb 2013

You probably just need to change "/" to "./"

Remember, the dot indicates elementwise operations.

This works:

```1./[1 2 3]
```

This does not:

```1/[1 2 3]
```

Answer by Youssef KHMOU on 12 Feb 2013

hi Kyle : try this :

``` Cpo = -0.65;
Mcr = linspace(1,8,20);
Minfin = linspace(0,1,20);```
``` Ccrit = (2./(1.4*Mcr.^2)).*(((((2+(.4*Mcr.^2))./2.4)).^3.5)-1);
%keyboard,
Cp = Cpo./(sqrt(1-Minfin.^2));```
``` figure,
plot(Mcr,Ccrit),xlabel('Critical Mach number'), ylabel('Critical Pressure coefficient value')
figure,grid
plot(Minfin,Cp,'r'), xlabel('object mach number'), ylabel(' Pressure coefficient' );grid```

## 1 Comment

Youssef KHMOU on 12 Feb 2013

The erro exist only in power operator "^" you have to use element-wise ".^" .

Answer by Kyle on 12 Feb 2013

Thank you everyone, got it working. @ Youssef KHMOU, you are a saint thank you